This invention relates to an oxide cathode emitting thermions. In particular, it relates to an oxide cathode in which the formation of interface layer is inhibited and troubles such as peeling of an oxide layer from the surface of base metal plate are prevented, as well as to a process for producing said oxide cathode.
Since an oxide cathode can emit thermions with a high efficiency at a relatively low temperature, it is widely employed in general uses, such as an image receiving tube in television. It is important for the oxide cathode to keep the emission of electron stable for a long period of time.
A conventional oxide cathode is constructed in that a layer of electron emissive alkaline earth metal oxide such as BaO alone, or a mixture of BaO, SrO and CaO in the form of composite substance (solid solution), is formed on a base made of an alloy comprising Ni as a main component and one or more reducing elements such as Mg, W, Si, Al, Zr and the like. When heated at a temperature of about 800.degree. C., it emits thermions. It is considered that the reducing element present in the base metal continuously reduces BaO, and the Ba thereby liberated functions as a center of the emission of thermions. Accordingly, if this reaction begins to lose its stability, electron-emitting characteristics of the cathode deteriorate. A cause of the deterioration seems to be at least partly attributable to the formation of a so-called interface layer of, for example, Ba.sub.3 WO.sub.6 between the base and the alkaline earth metal oxide layer.
The occurrence of the interface layer is particularly marked when W or Mo is incorporated into the base metal in order to enhance strength of a cathode at high temperatures and thereby to prevent its deformation, or in order to enhance its specific electric resistance and thereby to make it suitable for a directly heated type cathode. Also, it is frequently experienced that the formation and disappearance of the interface layer of, e.g., Ba.sub.3 WO.sub.6, induce peeling of the alkaline earth metal oxides from the base which ultimately results in destruction of cathode itself. One of the techniques for preventing this peeling is to roughen the surface of base by fusing powdered Ni or metal powder having the same composition as the base on the surface of base to be contacted with the oxide layer. However, the formation and disappearance of interface layer is so powerful as to destroy even the roughened surface and to make the roughened surface effectless. In conclusion, it is the most effective means for obtaining stable electron emitting characteristics to suppress the formation of interface layer to the utmost.